I am sure that as a web designer you always need open source application to help you in your work. If I am not right tell me and I will stop here. However, I am sure that I am right because those who work in the web design domain know that an open source is great not only because it is free but also because of the passionate and great community that forms around it.
Anyway, here you will find 30 open source application that can help you in your work anytime.
KompoZer is a web authoring system with an intuitive user interface, an integrated FTP client, and support for all the important operating systems.
Bluefish Editor is a fast editor created for web programmers and editors. It comes with a wizard for HTML documents. I for one consider that this wizard is very useful. It also has amazing project management characteristics, a powerful and useful search-and-replace option to bath-replacing the source code quickly.
Quanta Plus is a powerful IDE web development. It is very extensible and it can run custom script. It can boast with a great visual CSS editor. This editor has syntax auto-completion and code hints.
Amaya is another free open source web editor I recommend you. It is also a web browser developer. At first it started as a CSS/HTML editor and in time it has grown in an editor used for systems based on XML like MathMl or SVG.
CSSED is a CSS editor based on GTK2 that comes with a plug-in system for the utmost extensibility. In my opinion you can try it. You may find it a great application.
If you intend to create HTML web pages, I recommend you to use Open BEXI HTML Builder.
SeaMonkey is another open source, I want to recommend you. It is all-in-one web browser with a simple but powerful HTML editor for editing in-browser source code.
In your work you must often prototype and storyboard your designs, or you must create flowcharts for the navigation schemes. In this case, Kivio is a great flowchart/ diagramming editor.
Dia is a diagramming tool that can be very useful when you intend to create flowcharts or diagrams.
Use Skencil for illustration or diagrams. It allows you to save all your compositions in an ESP format that is recognized by almost all vector editing applications. For this task, you can use OpenOffice Draw, too.
For painting and editing, I suggest you to use mtPaint.
If you want to edit fonts for your design, I suggest you to try FontForge, a great open source font editor.
ImageMagick allows you to compose images in formats based on bitmaps. Its great feature is that it let you save hundreds of formats, including web formats as JPG, PDF, GIF and SVG.
Use HTML TIDY if you want to fix and optimize HTML syntax.
Other open sources that can be of a great use for you are: Cyberduck, an excellent open source for Mac OS, FileZilla a well known open source FTP client; WinSCP, CamStudio, Greenshot, ZScreen, Firebug, Xournal and Gallery. They are all useful open sources that are worth to be used in your web design work.